Bistable spring construction for a stent and other medical apparatus

ABSTRACT

The present invention is directed to bistable cells and their use in devices, particularly medical devices such as stents, clamps and valves. An expandable stent formed of a plurality of bistable cells is described. The stent has two or more stable configurations, including a first stable configuration with a first diameter and a second stable configuration with a second, larger diameter. A valve comprising a bistable cell for use in eliminating incontinence is also disclosed.

BACKGROUND OF THE INVENTION

There are several kinds of stents on the market with either balloonexpandable or self expanding function. Balloon expandable stents aregenerally made from a material that can easily be plastically deformedinto two directions. Before insertion, the stent is placed around theballoon section at the distal end of a catheter and pressed together toreduce the outer dimensions.

As soon as the stent is brought into the body in the proper axialposition it can be expanded and thereby plastically deformed by pumpingup the balloon. In this final position, the stent is at its largestdiameter and should function to support the surrounding tissue,preventing an undesired shape change into a much smaller diameter, atleast locally.

Therefore, the stent needs to have sufficient rigidity in the radialdirection, but also some flexibility in the axial direction when it isin the final position. Further, the amount of material should be assmall as possible and in the inner surface of the stent should notobstruct the flow through the channel (e.g., for blood) or cause toomuch turbulence.

Problems that generally occur with these stents are as follows: Aftercompressing the stent to its smallest diameter around the balloon, thestent will always have some elastic spring back to a slightly largerdiameter, which can cause problems when the catheter is brought into thepatient's body. In addition, the axial friction between balloon andstent can become so small that the stent slips off the catheter.Further, a larger size stent is typically a disadvantage.

A further problem is the so called recoil of these stents. This meansthat after expansion by the balloon pressure, the outer diameter willalways become slightly smaller as soon as the balloon is deflated. Thisdegree of recoiled can be as much as 10%, which can cause migration ofthe stent.

A different type of stent is made of a more or less elasticallyexpanding structure, which has to be held on the catheter by someexternal means. An example of this type is a stent that is held in itsconstrained state by a delivery sheath, that is removed at the momentthat the stent should deploy to its natural form.

Some of these stents are made of shape memory material with eithersuperelastic behavior or temperature sensitive triggering of theexpansion function.

A disadvantage of these self-expanding stents is the need for thedelivery sheath, causing a larger insertion diameter. The removal of thesheath also requires a sheath retraction mechanism, which has to beactivated at the proximal end.

Most stents of both types further have the disadvantage of relativelylarge length change during expansion and a poor hydrodynamic behaviorbecause of the shape of the metal wires or struts.

Another disadvantage of some stents is the positive spring rate, whichmeans that further expansion can only be achieved by higher balloonpressure.

The construction of prior stents is typically made in such a way thatthe external forces, working on the stent in the radial direction,merely cause bending forces on the struts or wires of the structure.

For example, a unit cell of a Palmaz-Schatz-stent, as produced byJohnson & Johnson Interventional Systems or the ACT One Coronary stent,produced by Progressive Angioplasty Systems, Inc. has in its collapsedcondition a flat, rectangular shape and in its expanded condition a moreor less diamond-shaped form with almost straight struts (Palmaz-Schatz)or more curved struts (ACT-One).

The shape of the unit cell of such stents is typically symmetrical withfour struts each having the same cross section. In addition, the loadingof the cell in the axial direction will typically cause an elastic orplastic deformation of all of the struts, resulting in an elongation ofthe unit cell in the axial direction. These unit cells have a positivespring rate. In stents based upon these unit cells the stability againstradial pressure is merely dependent on the banding strength of thestruts and their connections.

BACKGROUND OF THE INVENTION

In this patent application a new type of stent is described with a unitcell, having a negative spring rate and a bistable function. Such a unitcell can also be used in other medical applications. This means that ithas two configurations in which it is stable without the need for anexternal force to hold it in that shape. The unit cell is formed usingat least two different sections. One section is less pliable than theother one and acts a relatively rigid support that hinders the shapechange of the more pliable section in one direction. In the otherdirection the pliable section can be deformed, but because of theopposing force from the rigid section, the stability of the pliable orflexible section is strongly increased.

External forces in a direction perpendicular to the most pliable sectionare distributed to the rigid section and the cross section of thepliable section is merely loaded in compression mode. This makes theconstruction much stronger than prior stents. In prior stents, allstruts have generally the same cross section and mechanical propertiesand are merely used in the bending mode.

The construction of a stent, based upon this unit cell results in anapparatus, that can easily be elastically compressed around the balloonby finger pressure.

Below a certain critical diameter, the present stent snaps further to astable, smallest diameter, thus holding the deflated balloon firmly onto the surface of the catheter, with an insertion diameter that is assmall as possible. An additional sheath is not required, but may be usedfor extra safety.

After the stent has been brought into the patient's body at the properaxial position, the balloon can be inflated until the stent reaches itscritical elastic equilibrium diameter. Slightly above this diameter thestent automatically expands further to its final largest diameter, whereit reaches its maximum stability against radial pressure. The designenables a constant length large expansion ratio, a reliableexpandability and/or a small surface ratio.

A further embodiment of this invention is the possibility of a kind ofstepwise expanding stent with a range of stable diameters.

Another part of the invention is a stent with several external diametersalong its length, to adapt as good as possible to the shape of the bodycavity where it is placed.

Another part of the invention is the possibility to modify the stressand strain pattern in the unit cell by means of a heat treatment in sucha way, that the force displacement characteristic of this unit cellbecomes asymmetrical or even exhibits a monostable instead of a bistablefunction, either with the expanded diameter or the collapsed diameterbeing the most stable condition.

Another embodiment of the invention is the modification of the geometryof the cross section of some struts to achieve the symmetric orasymmetric bistable or monostable force against displacementcharacteristics of a unit cell.

Another part of the invention is the use of one or more unit cells inother medical applications such as, for example, an expander or a clip,either to spread a body cavity open or to clamp or hold a body part orsome body tissue.

The invention is also directed to the use of the inventive stents inconjunction with inventive expander rings to join together two vessels.

The invention is also directed to a bistable valve having a snap-actionbipositional unit cell and uses for the same, in particular, to preventincontinence.

The invention is also directed to multistable cells and their use inmedical devices.

Description of the Construction

The construction of the present stent includes a series of elements withan arrangement of unit cells that enable the stability in a special way.Each unit cell exists out of at least two distinct, mechanicallyconnected sections with different mechanical behaviors. One section actsas a relatively rigid support for the more flexible counteractingsection. The more flexible section is responsible for most, if not all,of the expansion of the stent. There are several ways to manufacture astent based upon this principle and it can be made from severalmaterials, like polymers, composites, conventional metals or shapememory alloys with superelastic behavior or with temperature sensitivebehavior.

It can be made from an arrangement of wire or strip, welded together atspecific places. Another possibility is metal deposition in the desiredpattern onto a substrate or the use of sintering of prealloyed powder.

A further method is making the stent from a tubular shaped startingmaterial, with a pattern of slits or slots made in the wall by means ofetching, grinding, cutting (e.g., with a laser, water, etc.), sparkerosion or any other suitable method. The pattern can also be made in aflat plate and then welded, brazed or crimped to a more or lesscylindrical shape or a cylindrical mid section with two conical endswith larger diameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C show the principle of a bistable mechanism;

FIG. 2 shows the force-displacement characteristic of the mechanism ofFIG. 1;

FIG. 3 shows another bistable mechanism with an asymmetric bistability;

FIG. 4 shows the force-displacement characteristic of the mechanism ofFIG. 3;

FIG. 5 a shows an inventive tubular stent in the stable, fully collapsedconfiguration;

FIG. 5 b shows an inventive tubular stent in the stable fully expandedconfiguration;

FIG. 6 shows a part of a stent with one bistable unit cell, drawn in thestable expanded shape;

FIG. 7 shows the part of the stent of FIG. 6 near its elastic bistableequilibrium position;

FIG. 8 shows the part of the stent of FIGS. 6 and 7 in its stablecollapsed shape; and

FIG. 9 shows a larger section of the stent of FIGS. 6 and 8, showingsome unit cells in the collapsed shape and some unit cells in theexpanded shape.

FIG. 10 shows an inventive stent formed of a plurality of smallerinventive stents joined together with flexible connectors.

FIG. 11 shows a partially expanded inventive stent having more than onetype of bistable unit cell;

FIG. 12 shows an inventive stent having a range of diameters along itslength;

FIG. 13 shows an inventive expansion ring in expanded state;

FIG. 14 shows the expansion ring of FIG. 13 in contracted state;

FIG. 15 shows an inventive stent joining two vessels together andfurther secured with inventive expansion rings, the stent exterior tothe vessels;

FIG. 16 shows a cross-sectional view of FIG. 15 along section line16-16;

FIG. 17 shows an inventive stent joining two vessels together, the stentinterior to the vessels;

FIG. 18 shows two vessels joined together with an inventive expansionring and a clamp

FIG. 19 shows a bistable valve in the closed position;

FIG. 20 shows the bistable valve of FIG. 19 in the open position;

FIG. 21 a shows a multistable cell in the fully contracted state;

FIG. 21 b shows the multistable cell of FIG. 21 a in the fully expandedstate;

FIG. 22 a shows another multistable cell in the fully contracted state;

FIG. 22 b shows the multistable cell of FIG. 22 a in the fully expandedstate;

FIG. 23 shows several unit cells as shown in FIGS. 21 a,b joinedtogether and in the fully expanded state;

FIG. 24 a shows several unit cells as shown in FIGS. 22 a,b joinedtogether and in the contracted state;

FIG. 24 b shows the interconnected cells of FIG. 24 a in fully expandedstate;

FIG. 24 c shows the interconnected units cells of FIG. 24 a in theprocess of expanding; and

FIG. 24 d shows several strips of interconnected cells as in FIGS. 24a,b joined together and in the process of expanding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the principle on which the stent is based, FIG. 1 a shows arod 1 with a length L, which is compressed in its axial direction unit;it reaches its buckling stress. Then the central part of the rod willband out in a sidewards direction, either to position 2 or 3 (dashedlines in FIG. 1 b). When the axial displacement L of the ends of the rodis held stable by external clamps 4, it is possible to move the centralsection of the rod between the two stable positions 2 and 3. Thismovement is in a direction X, perpendicular to the original length axisA-A of the rod. All positions between the stable positions 2 and 3 areunstable. In FIG. 1 b the central part of the rod has to rotate over anangle 13 before the rod can be moved in direction X. FIG. 1C shows asecond order curvature in rod 1, which occurs when the rotation overangle 13 is opposed by clamping the central part of rod 1 andmaintaining this part parallel to the axis A-A.

FIG. 2 shows the force F as a function of displacement X, with Xdisplayed in the horizontal direction. The rod is moved from the upper 2to the lower 3 stable position of FIG. 1. The force increases rapidlyfrom zero to Fmax. At that moment the onset of either the first orsecond order curvature of FIGS. 1 b and 1 c is reached. Furtherdisplacement in direction X costs less force, because this spring systemhas a negative spring rate. The force even becomes zero in the midposition and further movement occurs automatically. It can be seen inFIG. 2 that the system is completely symmetrical and the force needed tomove back from the lower to the upper position has the samecharacteristic.

FIG. 3 shows rod 5, which will have an asymmetrical force displacementcharacteristic, because it already has a preset curvature, even in theunloaded position, where the length is already L-L. This can be achievedby prior plastic deformation, heat treatment or the use of anasymmetrical geometry of the cross section of the rod (not shown). Therod 5 in FIG. 3 can be mounted between two clamps on a length L-L, andif it is elastically deformed in the same way as the rod in FIGS. 1 band 1 c, it will have a different stress distribution in the crosssection in end position 2 and 3, compared to the rod of FIG. 1. Thismeans that the rod has become a preferent unloaded table position, shownin FIG. 3.

FIG. 4 shows the asymmetrical force-displacement characteristic of theprecurved rod of FIG. 3. The initial displacement form the stable upperposition needs a starting force F1 and if the rod is in its table lowerposition the starting force in the opposite direction is only F2, beingsmaller than F1. Force F2 can be made as small as desired, even zero ornegative, but needs to have a positive value if stability of the lowerposition is required.

FIGS. 5 a and 5 b show the general appearance of an inventive tubularstent in fully contracted and fully expanded configuration respectively.The stent, in its fully contracted state shown generally at 50 and inits fully expanded state shown generally at 60, is comprised of aplurality of interconnected bistable unit cells (shown in the expandedstate at 64 in FIG. 5 b). The bistable unit cells are formed from afirst relatively rigid segment 52 (66 in FIG. 5 b) and a secondrelatively flexible segment 54 (68 in FIG. 5 b), joined together at ends70 and 72. Second relatively flexible segments 68 are interconnectedwith adjacent relatively rigid members 66. Adjacent cells in thelongitudinal sense (the longitudinal axis is denoted by referencenumeral 75) are joined at ends 70 and 72. By applying a uniform radiallyoutward or inward force, the stent may be switched directly from a fullycontracted to a fully expanded configuration or vice versa.

FIG. 6 (corresponding to inset 6 in FIG. 5 b) shows a small part of astent such as that shown in FIG. 5 which uses the bistable function of aunit cell, according to the present invention. The drawing shows ahorizontal line A-A, which is parallel to the central axis of the stent.There are two series of sinusoidal segments with distinct size (see alsoFIG. 9 for an overview). The segments 7 and 9 have a relatively largecross section. Only segment 9 is shown entirely. The segments 9 and 10have a relatively smaller cross section, and here only segment 8 isentirely shown. The segments are interconnected for example welded, atjoints 11 and 12.

Because of the difference between the cross section of segment 8 and 9,the deformation force of segment 8 is much lower than for segment 9.Therefore, segment 9 can be considered as a relatively rigid clamp, likethe clamps 4 in FIG. 1 b opposing relative displacement between thejoints 12 in the axial direction, parallel to axis A-A. In contrast,segment 8 acts as a flexible rod, like rod 1, described in FIG. 1 or rod5, described in FIG. 3. This combination of segments 7 and 8 or 9 and 10defines a unit cell, acting as a bistable spring system with aforce-displacement curve F-X like the described curves of FIGS. 2 and 4,depending on the unloaded condition and geometry of the segments.Alternatively, instead of using segments or struts of differentdiameter, the segments can have the same diameters (i.e., crosssectional area) and exhibit different strengths or rigidity and stillaccomplish the same effect. One way to obtain such differences instrength or rigidity would be to use different materials for thesegments. Another way would be to use the same material, like a metal,for all the segments but selectively strengthen (e.g., by heat treating)those segments that need to be rigid. It should be noted that heattreatment will not strengthen all materials. Nitinol, for examplebecomes more pliable as a result of heat treatment. This property ofNitinol can be exploited, however, to render one section of Nitinol morepliable relative to a second, non-heat-treated section of Nitinol.

FIG. 7 shows the same part of the stent (as depicted in FIG. 6) near theelastic equilibrium position. Segment 8 has bene deformed into thedirection X, caused by force F, but segment 9 has almost its originalshape, because of its larger rigidity.

FIG. 8 shows the same unit cell of the stent of FIGS. 6-7 after it hasbeen pressed through the elastic equilibrium position. It automaticallysnaps into its stable position of FIG. 8. This snapping force can bestrong enough to hold a deflated balloon tight on the catheter shaft(not shown), depending on the mechanical characteristics (e.g., thestrength) of the material(s) used to make the segments. With thegeometry shown in these figures, the segments 8 and 9 fit closetogether, taking up a minimum amount of space when the stent is in itssmallest stable diameter.

FIG. 9 shows a section of the stent of FIG. 5, flattened forillustrative purposes, showing several flexible segments in thecollapsed stable shape (segments 14, 18 and 20) and one segment element16 in the expanded stable shape. Segments 13, 15, 17, and 19 arerelatively rigid segments and substantially maintain their originalshape. The distance between two relatively rigid segments is shown as(h) in the collapsed stable shape and (H) in the expanded stable shape.The value of the displacement (H-h) in the direction X depends on theheight of an expanded unit cell or amplitude of the segments and thesize of the connecting joints. The described part of the stent is shownas a flat surface, but it may be clear that a cylindrical stent such asthat shown in FIG. 5 is shaped if segments 13 and 20 are directlyconnected to reach other with joints 21. In other words, the stent isshown separated along the joints 21 and in a flattened condition.

The range of stable diameters of the stent changes with the value(H-h)/π, each time that a flexible segment snaps from the collapsedstable position to the expanded stable position. The result is a stentwith an extremely rigid surface at all diameters being able to withstandthe external forces better than with conventional stents. In the lengthdirection, the flexibility of the stent can be increased bydisconnecting several unit cells from their neighbor unit cells, forexample, by cutting the center of one or more joints while maintainingthe several joint pieces as joints.

Another method to increase flexibility is to change the geometry ofseveral sections of the unit cells in the length direction from therelative flexible to the relative rigid shape several times along thetotal length of the stent. In other words, referring to FIG. 9 one ormore or each of the segments 13-20 could be constructed with larger andsmaller diameter (or otherwise flexible and rigid) sections whichalternate after each joint 21.

Another possibility, as shown in FIG. 10 is the use of a series of shortmultistable stents 100 aligned lengthwise end to end and connected withflexibility joints 104 having the same or a different geometry orconfiguration as the joints forming individual unit cells.

The scope of the invention should include all types of material. One ofthe most interesting materials is superelastic Nitinol, because of itslarge elastic strain, well defined stress values, caused by theirplateau stresses and the possibility to define the desired curvatureinto the metal by means of a heat treatment. A stent of Nitinol can bemade by forming slits or slots in a tube, while in its collapsed orsmaller stable diameter. The slotted tube is then expanded by a separateshaping tool and heat treated on this tool to define the expanded stablediameter as the unstrained shape.

In a more general sense, the present invention is directed to a devicehaving a plurality of stable configurations. The device is comprised ofa plurality of interconnected multistable cells. The cells include oneor more relatively rigid sections and one or more relatively flexiblesections interconnected so as to define a cell structure in the form ofa multistable spring system having a plurality of stable configurations.In a preferred embodiment, the cells comprise a first arcuate memberhaving first and second ends and a second arcuate member having firstand second ends, the first end of the first member in communication withthe first end of the second member, and the second end of the firstmember in communication with the second end of the second member. Itshould be noted, however that members need not be rigorously arcuate.Other shaped members, including relatively straight members arecontemplated as well.

The invention, in particular, contemplates bistable cells, that is cellshaving two stable configurations. In one such cell, the distance betweencorresponding points on the first and second sections is larger in thefirst stable state of the cell than in the second stable state of thecell. The cells themselves are constructed and arranged so that thedevice itself is at least bistable and possibly multistable. One suchdevice, a cylindrical stent having two or more configurations with aninitial diameter size and a final larger diameter size has beendescribed above. However, mutistable stents are also contemplated. Thus,for example, a stent may be constructed in which the cells are designedand arranged to provide a range of diameters in step-wise fashion. Onesuch way this may be accomplished would be to employ several differenttypes of cells in the stent, each type of cell having a different springconstant so that depending on the amount of force used, the stent wouldassume a different diameter. Such a stent in a partially expanded stateis shown schematically in FIG. 11. A partially expanded stent is showngenerally at 120. The stent is comprised of relatively rigid segments123, 127, 131 and 135 which substantially maintain their original shape,and relatively flexible segments 125, 129, and 133. The segments areinterconnected, with joints 122. As depicted, first flexible elements125, and 133 are in an expanded configuration while second flexibleelement 129 is in a contracted configuration. By applying a radiallyoutward or tangential force, flexible element 129 may be flipped to itsfully expanded configuration resulting in a stent (not shown) with alarger diameter. As shown in FIG. 11, cells 138 are larger than cells136 even in the contracted state. First flexible elements 125 and 133are characterized by a different degree of flexibility than secondflexible element 129.

Another form of stent, as shown generally at 150 in schematic FIG. 12,has an first diameter at a first end 152, a second diameter at a secondend 154 and one (or more) intermediate diameters in a region 156 betweenfirst end 152 and second end 154, the intermediate diameter differingfrom the first and second diameters. The interconnected cells in such astent, as shown generally at 150 in FIG. 12 may all have the same forceconstant and hence be openable all at once with the application of thenecessary force or there may be several different types of cells, eachwith their own force constant. In order to achieve the multiplicity ofdiameters, cells of differing sizes may be used. In one embodiment ofthis type of stent, the first and second diameters are the same while inanother embodiment, the first and second diameters differ.

The present invention is also directed to a method of implanting anexpandable stent having a plurality of stable configurations. The methodcomprises the steps of applying the stent to an expanding means on acatheter, delivering the stent to a desired bodily location, expandingthe expanding means so as to expand the stent from a first stableconfiguration to a desired second stable configuration, the secondstable configuration having a larger diameter than the first stableconfiguration, and deploying the expanded stent at the desired bodilylocation. The expanding means may be a balloon, a mechanical device onor in the catheter, a heat source where the cells can be induced tochange states by heating or any other suitable expanding means. Thestent may be applied to the balloon in the first stable configuration ormay be applied in the second stable (expanded) configuration during theapplying step. In the latter case radially inward pressure may beapplied to the stent so as to urge the stent into the first stableconfiguration to snap it onto the catheter. Where the stent hasadditional stable states, the stent may be applied to the balloon in anintermediate stable state in which the diameter of the stent isintermediate between the diameter in the first state and the diameter inthe second state. Again, the stent may be locked on the expanding meansby further applying a radially inward pressure.

A further object of the invention is the use of a single bistable unitcell as an expander (expansion ring), that can be brought into a narrowplace and then triggered to snap back into its expanded stable shape. Asshown in FIG. 13 an expansion ring shown generally in its expanded stateat 250 consists of a first rigid member 254 having first 258 and second262 ends and a second more flexible member 266 having first 270 andsecond 274 ends. First end 258 of first member 254 is connected to firstend 270 of second member 266 and second end 262 of first member 254 isconnected to second end 274 of second member 266. FIG. 14 depicts theexpansion ring of FIG. 13 in its contracted state. Second member 266 isseen to be in a second stable position.

Another object of the invention is the use of a single bistable loop(unit cell) as a clip, that can be used to clamp on an artery, fallopiantube or any other body part, to close or hold it for some time. For sucha clip it may be desirable to define the collapsed stable shape as theunstrained shape, because the collapsed stable shape has to be the moststable one. In the collapsed state, the clip would resemble thecollapsed expansion ring of FIG. 14. A triggering means would be used inconjunction with the clamp to switch the bistable loop from one state toanother. The triggering means may be pneumatic, hydraulic, mechanical,thermal or electromechanical means. Examples of such triggering meansinclude a human hand applying force to the bistable loop, and theapplication of heat to the loop. Other triggering means include pullingon the device, pushing on the device, bending the rigid section of thedevice or release a restraint holding the flexible member in place.

Another part of the present invention involves constructions between oneor more ring-shaped elements according to the present invention,combined with a tubular sleeve that is reinforced or held open with suchelements. An example is a so-called graft stent made of a polymer withone or more expansion rings. The expansion rings may consist of theabove-described bi-stable cells. The surface of the stent comprises askin mounted on the expansion rings. In mounting the skin, the skin maysurround, be in or between the expansion rings. The skin may be human oranimal skin, a polymeric material or any other suitable bio-compatiblematerial. Such a stent may comprise one or more expansion rings, such asa first expansion ring at a first end of the stent and a secondexpansion ring at a second end of the stent. The stent may be ofconstant diameter along its length or may have a first diameter at thefirst end and a second diameter at the second end.

The present invention is also directed to a stent having an unexpandedconfiguration and an expanded configuration, and comprising a pluralityof generally longitudinal, wave-like first members characterized by afirst wavelength, and having peaks and troughs and a plurality ofgenerally longitudinal wave-like second members characterized by asecond wavelength, and having peaks and troughs. The wavelengths of thefirst and second longitudinal members are substantially equal. Thesecond members are capable of stably assuming two positions, a firstposition corresponding to the unexpanded configuration in which thefirst and second members are in phase and a second positioncorresponding to the expanded configuration, in which the first andsecond members are 180° out of phase. The first members are more rigidthan the second members. The first and second longitudinal members aredisposed on the surface of the stent such that the longitudinal firstand second members alternate. In the unexpanded state, each peak of eachfirst member is connected to one adjacent peak of a second member in aregion of attachment and each trough of each first member is attached toone adjacent trough of a second member in a region of attachment, as canbe seen from FIG. 8. The regions of attachment are separated along thelongitudinal direction by one wavelength. The so described stent can besnapped from the unexpanded configuration to the expanded configurationby applying a radially outward force and similarly can be snapped fromthe expanded to the unexpanded configuration by applying a radiallyinward force. While such stents may be used internal to a bodily vessel,it may also be used external to vessels to join two vessels together.

The invention also contemplates a method of joining together two vesselscomprising the steps of delivering an inventive stent in an unexpandedconfiguration in a first stable state to a bodily site, expanding thestent to a second stable state, the diameter of the stent in the secondstable state exceeding that of the vessels to be joined and placing thestent over the vessels to be joined. The stent may then be contracted toa third stable state, the stent in the third stable state having adiameter intermediate between the diameters of the stent in theunexpanded state and in the second stable state. The stent may furtherbe secured to the vessel with the aid of one or more of theabove-described expansion rings (a bistable loop). One or more expansionrings, such as that depicted in FIGS. 13 and 14 or small clamping stents(such as that formed from the strip shown in FIG. 23) may be deliveredto each side of the stent in a contracted state and deployed so as toclamp the vessels between the ring(s). Multiple rings may be used foradditional clamping. As shown generally at 300 in FIG. 15, a firstvessel 304 and a second vessel 308 are joined together with inventivestent 312. Vessel 304 overlaps stent 312 in a first overlap region 316while vessel 308 overlaps stent 312 in a second overlap region 320.Vessel 304 is clamped between expansion ring 324 (shown in the expandedstate) and stent 312 while vessel 308 is clamped between expansion ring328 (shown in the unexpanded state for illustrative purposes only) andstent 312. the dotted lines associated with expansion ring 328illustrate expansion ring 328 in its expanded state. It should beadditionally noted that FIG. 15 provides a cut-away view of vesselsshowing the rings contained therein. FIG. 16 shows a cross-sectionalview of FIG. 15 along section line 16-16. Vessel 304 is shown sandwichedbetween stent 312 and expansion ring 324.

In another embodiment, as shown in FIG. 17, a first vessel 404 and asecond vessel 408 are joined together by a stent 412. First end 416 ofstent 412 rests in vessel 404 while second end 420 of stent 412 restswithin vessel 408. Optional clamps (such as a small portion of acollapsible inventive stent shown later in strip form in FIGS. 23) 424and 428 residing on the outside of vessels 404 and 408 clamp the stentto the vessel. Additional clamps may be used as needed.

In another embodiment, a combination of the embodiments of FIGS. 15 and17, the first end of the stent may protrude from one of the vessels andthe second end of the stent may extend over the second vessel. Again,clamps and expansion rings may be used to further secure the stent tothe vessels.

In another embodiment, as shown in FIG. 18, vessel 454 and vessel 458are held together by an expansion ring 462 internal to the vessel and aclamp 466, consisting of, for example, a small section of collapsiblestent, the stent chosen so that the diameter of the stent in a collapsedstate affords a snug fit with vessels 454 and 458 and expansion ring462. Either the expansion ring or the clamp, but not both, may bereplaced by a suitable support such as a rigid collar.

The invention also contemplates a method of joining together two vesselscomprising the steps of delivering an inventive stent in an unexpandedconfiguration in a first stable state to a bodily site, placing twobodily vessels over the stent and expanding the stent to a second stablestate, the diameter of the stent in the second stable state exceedingthat of the vessels to be joined. The diameter of the stent in thesecond stable state is preferably chosen so that the vessels fit snuglyover the stent. The delivery of the stent may be accomplished bydelivering the stent in an unexpanded configuration through a bodilyvessel and subsequently expanding the stent to rest snugly in thevessels to be joined (where a portion of the stent resides in a vessel),or by expanding the stent to its most expanded state, placing the stentover the vessel and then contracting the stent to an intermediate stateover the vessel. The collars and expansion rings mentioned above maysimilarly be delivered. Alternatively, the stent, collars and expansionrings may be delivered by surgically exposing the vessel in question.

The present invention is also directed to a bistable valve. The valve,as shown generally at 600 in FIG. 19 includes a snap-action bipositionalunit cell shown generally at 604 located within a conduit 606.Snap-action bipositional unit cell 604 consists of a (substantiallyarcuate) flexible member 608 having a first end 612 and a second end616. First end 612 is in communication with a triggering means 620 whichis supported, in turn by a support means 624 emerging from the innersurface of conduit 606. Second end 616 of flexible member 608 isanchored to stop surface 628 which extends across conduit 606. Supportmeans 624 and stop surface 628 must be sufficiently rigid to holdflexible member 608 in place and must be more rigid than flexible member608. Stop surface 628 extends substantially obliquely across conduit 606in oblique regions 630 and has a opening 632 within in longitudinalregion 634 to allow the flow therethrough of a fluid. Although opening632 is oriented along the longitudinal axis 636 of conduit 606, those ofordinary skill in the art will recognize other possible orientations ofthe opening and stop surface. Valve closure member 640, actuated betweenopen and closed positions by flexible member 608, is constructed andarranged so as to block the flow of fluid through opening 632 whenflexible member 608 is in the closed position. When flexible member 608is in the open position, as depicted in FIG. 20 valve closure member 640no longer obstructs opening 632, thereby allowing the flow of fluidtherethrough.

While triggering means 620 may be any suitable mechanical, hydraulic,pneumatic, or thermal based trigger known in the art at present or inthe future, in a preferred embodiment, triggering means 620 is apiezoelectric element. In operation, if the piezoelement shown in FIG.19 at 620 is not activated, valve closure member 640 is closed.Activation of piezoelement 620, as shown in FIG. 20 causes a smallshortening in the longitudinal length (denoted by Y in FIG. 15) ofpiezoelement 620 which in turn releases flexible member 608 from itsfirst position. With member 608 released, valve closure member 640 isfree to open under the pressure transmitted from the fluid. Member 608assumes a second, inverted, position, as depicted in FIG. 20 While thefluid pressure maintains member 608 in its second position, even in theabsence of any fluid, member 608 remains in its second position, asdepicted in FIG. 20 if the triggering is turned off and piezoelement 620assumes its original length. Valve closure member 640 may be closedagain, in the absence of fluid, by a subsequent triggering ofpiezoelement 620 allowing member 608 to transition to its second(closed) position which is the preferred position of member 608. Member608 has been treated to receive a preferred position as shown in FIG. 3.

The valve depicted in FIGS. 19 and 20 may be applied to medical andnon-medical devices. It is, in particular, an aim of the presentinvention to apply the inventive bistable valve to the control ofurinary incontinence. In a patient with incontinence, the abovedescribed valve may be implanted in the urethra using any suitable meansincluding the use of the above-described expansion rings to clamp thevalve to the urethra. Although the valve in the default state is closed,the valve may be triggered when the bladder is full, to void thebladder. Upon voiding the bladder, the valve may be triggered again toclose it. Another such application is to employ the inventive valve inconjunction with a shunt. The shunt may be activated by triggering thedevice and similarly may be closed by triggering the device.

Of course the valve may be used in other medical and non-medicalapplications as well.

In addition to the bistable unit cells disclosed above, bistable unitcells and more generally, multistable unit cells of other shapes arealso contemplated by the present invention. FIGS. 21 a and 21 b areschematic representations of another embodiment of an inventive hingedmultistable cell in its contracted and expanded states, respectively.The contracted cell, shown generally at 700, and the expanded cell,shown generally at 705, consist of four interconnected relatively rigidmembers. Two side members 709 are connected to opposite ends of topmember 713 via hinges 715. Side members 709 are connected at theiropposite ends to opposite ends of bottom member 717 via hinges 719.Preferably, the hinges are elastic or plastically deformable. The hingesmay be fixedly attached to the side, top and bottom members or may beintegral with these members. In the latter case, the hinges may beformed by removing material from the cell in the region of the hinges sothat the hinges are thinner or have a different geometry from the side,top and bottom members. In the process of transitioning from theexpanded to the collapsed state, bottom member 717 opens slightly. Thecell of FIGS. 21 a,b also has two additional intermediate states inwhich one or the other (but not both) of side members 709 and top member713 are collapsed downward.

A hexagonal hinged multistable unit cell is shown schematically in FIG.22 a in the collapsed state and in FIG. 22 b in the expanded state. Thecell, shown generally at 750, consists of top member 754 and bottommember 758, and upper side members 762. Two upper side members 762 areconnected to opposite ends of top member 754 via hinges 756. Upper sidemembers 762 are connected to bottom member 758 via hinges 768. Bottommember 758 is shaped like a ‘U’ with the two uprights of the ‘U’modified to lie at oblique angles with respect to the bottom part of the‘U’. As with the previously discussed inventive cells, hinges 756 and768 may be elastic or plastically deformable and may be fixedly attachedto the members or integral with the members. The hexagonal unit cellexhibits multiple stable states. In addition to the fully expanded andfully contracted states shown in FIGS. 22 a and 22 b, the hexagonal cellcan also achieve two intermediate stable configurations in which onlyone of the two upper side members 762 is collapsed inward along with topmember 754.

The above described hinged multistable cells may be used in any of theabove discussed applications e.g. to form stents, clamps, clips,expander rings, bistable valves.

In one such application a ring or stent is formed of the hinged cells ofFIGS. 21 a and 21 b. As shown in FIG. 23, a series of unit cells of thetype depicted in FIG. 21 are joined together so that the top member of acell forms a portion of the bottom member of an adjoining cell. Asdepicted, top member 814 of cell 810 forms a portion of bottom element818 of cell 820. Similarly, top member 824 of cell 828 forms a portionof bottom element 832 of cell 836. Although the ring or stent in FIG. 23has been cut for illustrative purposes, the two ends 840 and 844 arenormally joined together with a portion of lower member 848 of cell 852serving as an upper member for cell 856. The ring so formed has a rangeof stable stable states including a fully expanded state and a fullycontracted state. Where the individual cells are made identically, onlythe fully expanded states may be accessed by the application of auniform radially outward force to the stent in the fully contractedstate. It may serve as a clamp or collar, an expansion ring or a stent.Larger stents may be formed by interconnecting a plurality of suchrings.

Similar products may also be formed from other multistable units cells.FIGS. 24 a and 24 b illustrate one such possibility schematically inwhich hexagonal unit cells such as those shown in FIGS. 22 a,b may bejoined together to form a ring. The top member 884 of each cell 880 isjoined with a the bottom portion 886 or modified ‘U’ shaped bottommember 890. Although shown in strip form in FIGS. 24 a and 24 b, end 894can be joined to end 898 to form a ring. The strip of FIG. 24 a is shownin fully expanded state in FIG. 24 b. Adjacent cells 880 are seen intheir expanded state. For the sake of completeness, the hinges aredesignated 902. FIG. 24 c shows one cell 920 in the process of expandingand one already expanded cell 924. The cells 920 and 924 are joined atbottom member 928 and top member 932. Hinges are shown at 936. Multiplestrips may also be joined together so as to form a stent whose length isa multiple of the length of the unit cell. In such a case, upper sidemembers of adjacent cells would be joined together. This is illustratedin FIG. 24 d which, like FIG. 24 c shows cells 940 in the expanded stateand cells 944 in the process of expanding. Upper side members 948 areshown by dashed lines. Adjacent strips of interconnected cells 952 arejoined together by upper side members 948 as well as by oblique regions956 of bottom members 960.

It should be noted that the inventive devices of the present applicationmay be use on a temporary basis or on a permanent basis in the body.Thus, for example, permanent stents and clamps are contemplated, as areremovable stents and clamps.

It should further be noted that in expanding-some of the inventivemultistable cells, there may be components of expansion/contraction in adirection perpendicular to the direction of the force applied to expandthe cells.

Finally, for the purposes of this application, the term ‘multistable’ isintended to include ‘bistable’.

In the described drawings and text only some examples of differentembodiments have been given. While the stents of the present inventioncan appear similar to prior stents, the mechanical results arecompletely different due to the special combination of a rigid sectionand a more flexible section in the same unit cell. Of course there are,beside the illustrated sinusoidal shape many other possible basic shapesfor the unit cells, with similar characteristic behavior.

From the above disclosure of the general principles of the presentinvention and the preceding detailed description, those skilled in thisart will readily comprehend the various modifications to which thepresent invention is susceptible. It is intended for the coverage of thepresent application to include different geometries, differentconstructions and different combinations of one or more materials toobtain the same basic mechanical behavior as exhibited by the abovedescribed examples.

What is claimed is: 1-20. (canceled)
 21. A bistable valve comprising: 1)a conduit; 2) a stop surface extending across an interior of theconduit, the stop surface having an opening within; 3) bipositional unitcell, the unit cell including a substantially arcuate flexible memberhaving a first end and a second end, the first end in communication witha trigger, the trigger supported by a support emerging from an innerwall of the conduit, the second end anchored to the stop surface; 4) avalve closure member.
 22. The bistable valve of claim 21, wherein theflexible member has a first position corresponding to a closed positionand a second position corresponding to an open position.
 23. Thebistable valve of claim 21, wherein the valve closure member actuatesbetween open and closed position by the flexible member.
 24. Thebistable valve of claim 21, wherein the valve closure member closes theopening in the stop surface when the flexible member is in the closedposition.
 25. The bistable valve of claim 21, wherein the valve closuremember opens the opening in the stop surface when the flexible member isin the opened position.
 26. The bistable valve of claim 21, wherein theconduit is opened by triggering the trigger to allow the flexible memberto move between the closed position and the opened position.
 27. Thebistable valve of claim 21, wherein the conduit is closed by triggeringthe trigger to allow the flexible member to move between the openedposition and the closed position.
 28. The bistable valve of claim 21,wherein the trigger is a piezoelectric element.
 29. The bistable valveof claim 28, wherein the piezoelectric element restrains the flexiblemember.
 30. The bistable valve of claim 28, wherein the piezoelectricelement decreases in length upon introduction of a small current to thepiezoelectric element and releases the flexible member from its firstposition.
 31. The bistable valve of claim 21, wherein the support is arigid member relative to the flexible member.
 32. The bistable valve ofclaim 21, wherein the stop surface has two oblique regions with alongitudinal region therebetween, the opening disposed in thelongitudinal region.
 33. The bistable valve of claim 32, wherein theoblique regions are oblique relative to a longitudinal axis of theconduit.
 34. A bistable valve comprising a bistable cell for use ineliminating incontinence.
 35. A medical device comprising the bistablevalve of claim
 21. 36. The medical device of claim 35, wherein themedical device is chosen from a shunt or an expansion ring.
 37. A methodof controlling urinary incontinence comprising: 1) inserting a medicaldevice comprising the bistable valve of claim 1 into a portion of aurethra; and 2) optionally clamping the medical device in place byapplying a clamp to the outside of the urethra; wherein urine may bevoided by triggering the bistable valve so as to switch it from theclosed to the opened position, the valve being triggered so as to closefollowing urination.
 38. Method of operating a bistable valvecomprising: activating a trigger that releases a flexible member fromits first position; opening a valve closure member that forces theflexible member into a second position; and optionally activating thetrigger that allows the flexible member to transition to its firstposition.